1. Lipids:

•Bloor coined the termed lipid.

•These are esters of fatty acids and their derivatives; composed of primarily C, H and O.

•The amount of Oxygen (O) is considerably very less and the ratio of H:O is not 2:1.

•Besides carbon, hydrogen and oxygen small amount of phosphorus, nitrogen and sulphur are also present.

•The basic component of lipid is fatty acid.

•Many lipids are composed of both glycerol (tri hydroxy propane) and fatty acids

2. Simple Lipids:

•Simple lipids are regarded as esters of long chain fatty acids and alcohol, majorly glycerol with ester linkage.

•Such lipids are regarded as Triglycerides.

•The synthesis of triglycerides involve dehydration reaction and three molecules of water are released.

Structure of Lipids.

3. Fatty Acids:

•Fatty acids are organic acids with a hydrocarbon chain ending in carboxylic group (-COOH).

Saturated Fatty Acids Unsaturated Fatty Acids
General formula (Cn H2n O2) General formula (Cn H2n-2x O2)
They do not have any double bonds in their carbon chains They have one or more double bond in their hydrocarbon chain
They have higher melting points and are solid at room temperature They are liquid at room temperature due to presence of double bond
Examples: Palmitic acid, Stearic acid Examples: Oleic acid, Linoleic acid

Structure of Saturated and Unsaturated Fatty Acid

•Depending upon the number of fatty acid attached to glycerol, esters can be mono, di or triglycerides.

•The three fatty acids are similar only in few fats and are known as pure fats while as most of the fats have dissimilar fatty acid chains and are known as mixed fats "e.g. butter.

4. Waxes:

•Waxes are defined as highly insoluble esters of long chain monohydroxy alcohols except glycerol and fatty acids.

•Waxes have higher melting point than true fats.

• Waxes are resistant to hydrolysis as compared to triglycerides.

5. Conjugated Lipids or Compound Lipids:

•Lipids composed of fatty acids, alcohols and other compounds like phosphorus, amino nitrogen, carbohydrates.

•Compound lipids are majorly of two types i.e. phospholipids and glycolipids respectively.

(a) Phospholipids:

•These can be defined as triglycerides in which one fatty acid is replaced by a phosphate group.

•These are amphipathic in nature i.e. have both hydrophilic (H3PO4 and nitrogenous compound) and hydrophobic non-polar groups (fatty acids) and thus are capable to form bilayers in aqueous medium as shown below

Structure of Phospholipid.

→Examples of phospholipids:

(i) Lecithin:

•1 glycerol + 2 fatty acids + 1 phosphoric acid + choline (nitrogenous compound), choline is attached with phosphoric acid. Lecithin is found in yolk, brain, soybean membrane, oil seeds and blood. It acts as a lipid career in blood.

(ii) Plasmalogen:

• Plasmalogen is platelet-activating factor (PAF) released from basophils (WBC in vertebrates) to stimulate the blood platelets.

•It is found in vertebrate cardiac muscles, ciliate protists and certain cells of invertebrates.

(b) Glycolipids:

•Fatty acids + amino alcohol + one or more simple sugars, present in cell membranes, myelin sheath of nerve fibres and membrane of chloroplasts.

→ Examples of glycolipids are:

(i) Cerebrosides:

It is essential lipid of white matter of brain and myelin sheath of the nerve fibres.

(ii) Gangliosides:

•It is found in grey matter of brain, membrane of RBC's, spleen.

•They also have neuraminic acid.

(c) Lipoproteins:

•Phospholipids + proteins.

•Mainly found in blood, milk and egg yolk.

6. Derived Lipids:

•Derived lipids are derived upon hydrolysis of simple or compound lipids.

•These are complex in nature.

(a) Steroids

•These are tetracyclic in structure called as cyclo pentano perhydrophenanthrene nucleus.

Steroids are classified into two types based on the functional groups:

(i) Sterols:

•Alcoholic steroids are called as sterols

→Example: cholesterol (C27 H45 OH).

• Cholesterol and its esters are insoluble in water.

•Cholesterol is abundantly found in brain, nervous tissue, adrenal gland and skin.

•7-dehydro cholesterol occurs in skin is a provitamin.

•On exposure to ultraviolet radiation it transforms into cholecalciferol i.e. vitamin D.

(ii) Sterones:

•These can be defined as steroids carrying ketone group e.g. testosterone, adreno corticoids, ecdysone hormone of insects, Diosgenin obtained from yam plant (Dioscorea) is used in manufacturing of antifertility pills.

Structure of Steriod.

(b) Terpenes:

•Terpenes can be defined as lipid like hydrocarbons formed of isoprene units (C5 H8 ).

•It includes certain fat-soluble vitamins like A, K, E, carotenoids and certain coenzymes like coenzyme Q.

•Terpenes are also regarded as chromolipids.

•It is the most complex lipid in protoplasm.